Find a way to combine current skill system with learn-by-doing. Is this a survival game or a role playing game? The current system is cool, but doesn't exactly feel like "survival" when the point of survival is to avoid zombies.
Horde night is cool, but it needs work. Every 7 days is just tedious. I get that some people really enjoy that game style, but personally I prefer no horde, and a more constant 24/7 type of threat, like nightmare speed zombies with no daylight at all. Having said that, allow an option to set the length of the horde event rather than using the equation: 24 - daylighthours = hordehours. I set the horde to 0 zombies and the speed to walk, and since I use 0 daylight, I get a 24 hour period of walking zombies to give myself a break from real apocalypse conditions. (Blood moon is shiny compared to 0 daylight, so I justify it by saying it distracts the zombies. W/E IMO way more fun than dealing with a horde every 7 days.) More options than this for horde night though. Maybe consider adding a random range in which it chooses the night. Leave the first horde night on day 7, but make all following hordes land somewhat randomly around 7 day intervals. A zombie apocalypse running on clockwork doesn't sit right with me, and is also boring.
In order to play on a "high difficulty" there should be more options on the zombies. Increasing health of zombies to increase game difficulty is lame. LAME. I said it. LAME. That doesn't make it difficult. It makes it tedious. There should be options for speed, damage, health (even tho it's lame), amount of zombies, heat map multiplier, noise multiplier, individual settings for zombie types...
On that note: What's with all the vultures? They're zombies, but they are procreating at a high rate because of all the dead bodies? Did they procreate then die afterward? I find it hard to believe that a zombified animal could fly... really? Bring back the bees. Vomiting vultures? Cops are ok, but this is really pushing a lack of imagination here. I will never respect the current vultures. Nope.
Can anyone say .50 cal? Come on. Maybe some AR-15s, and bullets that can pierce. (Example, in RL AR-15 pierces, AK doesn't.)
Also different bullet calibers use different bullet casings... Not a big deal, just something that irritates me. Try to put a 9mm case on a .762 bullet in RL.
Fishing... for real. This should have been in A1.
Swords? I mean ♥♥♥♥... I have 4 in my house right now. Who doesn't use a sword in a zombie apocalypse? What country is this? What?
More focus on PvP and/or WAY more endgame content. The gun and armor mods are a good start. This game needs clans, and clan wars over resources, and unique items that give clans a reason to raid each other.
NPC factions that you can gain and lose points in. And like was mentioned by an earlier poster, set some traders to use factions.
Vending machines (also mentioned by someone else) shouldn't refill (unless at a trader) or take tokens (at all).
Zombie loot should be appropriate to zombie type (also mentioned by another poster) like it use to be... Why did that go away? Nurses give meds, workers give wrenches... It made sense, and it's not like those things need to be held back from the player until a certain point or anything. That stuff is day 1 content anyway.
Chicken coops. Pig pens. (also mentioned) Make them generate a lot of heat.
Bring caves back.
Bring parts quality back for vehicles. At least the engines, wheels, and batteries... That was a good system. (also mentioned)
Way Way Way more melee weapon options. WAY MORE.
A push to get zombies off you, possibly knock them down. Must use medium-heavy weapons with 2 hands. Using a pistol or knife would be useless.
Change the drop item button to a yell button that wakes up all the zombies nearby.

I usually just smack the front door, but yelling would be cooler.